The present invention relates to an exposure apparatus used for forming the phosphor screen of a color cathode ray tube.
The formation of a phosphor screen on an inner surface of a face panel during the manufacture of a color cathode ray tube includes a step of exposing a photosensitive resin film formed on the inner surface of the face panel in accordance with patterns of red, green and blue phosphor dots. Normally, a light absorbing layer, referred to as "black matrix", having phosphor dot forming regions in the form of holes is formed on the inner surface of the face panel and, thereafter, phosphor dots in three colors are formed on the dot forming regions. An exposure apparatus as described below is used to form such patterned black matrix and phosphor dots.
Specifically, the exposure apparatus comprises a lamp house including an extra-high voltage long arc mercury lamp serving as an exposure lamp, an optical lens arranged to oppose to the lamp house, and a light distribution filter arranged to oppose to the optical lens. An exposure beam emitted from the mercury lamp is refracted by the optical lens to be corrected such that its optical path coincides with the path of electron beams and is then transmitted by the light distribution filter to be projected upon the inner surface of the face panel through apertures of a shadow mask. As a result, a photosensitive phosphor surface film coated on the inner surface of the face panel is printed in correspondence with the aperture pattern of the shadow mask. The light distribution filter is a glass filter formed with a deposition film in a predetermined pattern on the surface thereof and corrects illumination distribution on the inner surface of the face panel.
The lamp house includes a window glass as a light-transmitting window provided at an upper portion (on the face panel side) thereof, and a straight tube type long arc mercury lamp is arranged in the lamp house. The lamp house has a structure in which cooling water is circulated to cool the mercury lamp. Further, a shading plate having a V-shaped slit is arranged above the lamp in the lamp house at a predetermined interval therefrom. In this case, the shading plate is arranged such that the longitudinal direction of the lamp is perpendicular to the longitudinal direction of the slit. The shading plate is secured to the lamp house by mating a slit locating hole thereof with a locating protrusion on the lamp house and by urging it using an O-ring and a locking ring with the window glass arranged above the V-shaped slit in a face-to-face relationship therewith.
A conventional shading plate is formed by using, for example, a darkened thin iron plate with a thickness in the range of 0.1 to 0.3 mm and by bending the region of the iron plate including the slit into a V-like configuration using a press process. The configuration of the V-shaped slit varies depending on the color cathode ray tube in which it is used. The bending angle defined by the two sides of the character V is normally within the range of 80 to 180.degree.; the length of the slit (the length of one of the sides of the character V) is within the range of 3 to 10 mm; and the width of the slit is within the range of 0.5 to 2.5 mm.
In the exposure apparatus having a lamp house in the configuration as described above, an exposure beam emitted from the exposure lamp passes through the V-shaped slit and thereafter exposes the photosensitive phosphor surface film on the panel inner surface through the apertures in the shadow mask. The position in the V-shaped slit where exposure beam passes through is the apparent position of the light source.
In such an exposure apparatus utilizing a lamp house which is formed in resemblance to a point light source by providing a shading plate bend in a V-like configuration in a face-to-face relationship with a straight tube type light source lamp, the directivity of the exposure beam emitted from the exposure lamp significantly varies depending on the distance between the exposure lamp and a V-shaped bottom portion of the shading plate and the bending angle of the shading plate.
Although the interior of the lamp house is in general cooled by cooling water, the shading plate is unavoidably heated by radiant heat because a high-output mercury lamp is used as the exposure lamp. Meanwhile, since the shading plate is formed by bending a thin plate, it has low mechanical strength and is therefore subjected to deformation attributable to thermal expansion due to heat generated by emission from the lamp for a long time. The deformation of the shading plate, i.e., changes in the angle and height of the V-shaped slit causes a change in the positional relationship between the exposure lamp and the slit and in a change in the angle of incidence of the exposure beam upon the shadow mask. When the phosphor screen is manufactured in such a state, the photosensitive phosphor surface film is exposed in positions shifted from predetermined positions, and this causes poor exposure of the phosphor surface.
Further, since the shading plate is secured by mating the locating protrusion provided on the inner wall of the lamp house with the locating hole of the shading plate, misalignment is likely to occur. In the case of misalignment of the shading plate, poor exposure of the phosphor surface occurs again as in the case described above.